Power is often needed to be provided to downhole electrical devices such as pumps and heaters. Electrical heaters may be used to heat the subterranean formation by radiation and/or conduction, or the heater may resistively heat an element. U.S. Pat. No. 6,023,554 to Vinegar et al., assigned to Shell Oil Company, describes an electrical heating element that is positioned within a casing. The heating element generates radiant energy that heats the casing. A granular solid fill material may be placed between the casing and the formation. The casing may conductively heat the fill material, which in turn conductively heats the formation. While a large number of patents are directed to methods of heating a subterranean formation, very few patents provide the necessary teachings to provide wellhead equipment capable of providing the high voltages likely to be needed to heat heavy oil formations, while maintaining well control in pressure environments. For instance, in U.S. Pat. No. 7,004,247 to Cole et al., assigned to Shell Oil Company, it is noted that, for heaters greater than about 700 m in length, voltages greater than about 2000 V may be needed for generating heat, compared to voltages of about 480 V that may be used for heaters having lengths less than about 225 m.
U.S. Pat. No. 4,716,960 to Eastlund et al., describes electrically heating of the tubing of a petroleum well by passing a relatively low voltage current through the tubing to prevent formation of solids. Isolated tubing hangers are known in the oil and gas industry. They are often used when providing an electrical connection to one or more downhole electrical devices such as pumps or electrical instruments. However, for the most part, the power is supplied by electrical cables, which are accommodated through a tubing hanger by feed through connectors which provide electrical isolation from the hanger. Exemplary patents relating to insulated tubing hangers include U.S. Pat. No. 4,923,006 to Hartmann et al. and U.S. Pat. No. 6,763,882, issued Jul. 20, 2004 to Demny et al. U.S. Pat. No. 5,280,766 to Mohn describes a subsea wellhead system in which concentric tubular conductors with insulating sleeves therebetween are used to provide power to a pump. Few details for providing the electrical connection at the wellhead are provided.
In general, the prior art patents are not directed to the unique problems associated with the provision of high voltage to downhole electrical devices through wellhead tubing strings under pressure-containing and electrical isolating conditions. For instance, the current needed to run downhole instrumentation, pumps or even to heat a tubing to prevent a wax build up, is minor compared to that needed to run downhole heaters in heavy oil reservoirs.
As well, the patents relating to isolated tubing hangers suffer the disadvantage of not providing a feature for making an electrical connection at the wellhead when the well is under pressure.